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Old 06-13-2012, 07:10 PM   #29
More than one rivet loose
 
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Originally Posted by Chrispate View Post
Ye you can with a specialized welding called TIG tungsten inert gas welding. The toughest to master in my opinion an expensive machines, but possible. Side note the skin in my mind is way to thin to TIG weld comfortably.
I personally find TIG easier then Gas welding.
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Old 06-13-2012, 07:14 PM   #30
More than one rivet loose
 
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I agree with Splitrock it is a design issue not a materials issue.

Perry
I plugged the opening in the front of the A-frame after spraying copious amounts of corrosion inhibitor.
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Old 06-13-2012, 07:47 PM   #31
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It is true that some heat treated aluminum alloys can not be welded without destroying their strength. If you went with a welded frame, you would not want to use these alloys. Lots of aluminum airplane frames are made from welded aluminum space frames. The easiest aluminum frame to made would be from aluminum I beams of a weldable alloy. An alternative frame material would be extruded rectangular closed cross section beams. The cross members would be either welded to it or bolted. The cost would certainly be higher that the steel frames that are used now and likely more costly than hot dip galvanizing the current steel frames. The current steel frames are custom made for Airstream by an outside supplier. The galvanizing would be done before they are delivered to Airstream. I imagine that Airstream could make a galvanized frame could be a customer selected option on a special order trailer.
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Old 06-13-2012, 07:58 PM   #32
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I've read here in the forum of those who did a skin-off restoration of their Airstreams using Marine Grade plywood for their floors. In my opinion, has to be a better way to go, and one that I'll be using when I rip up carpet and floors in my Airstream, but first, gotta get rid of that water leak......lol
Thanks, Derek
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Old 06-13-2012, 09:58 PM   #33
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Dwight. I'm thinking a 6" I beam with a 5/8 web and a 4'" flange. The 4" flange would give you lots of fastening surface and the web in my opinion would be more than what is needed. This is all in my head and I've done no calculations and in all probability I won't.

What about a light weight truss system with knee bracing. With enough money I could design one that would last forever.

I agree that the steel framing being used now is adequate and safe. Thermal diffusion galvanizing would be I think a better process than hot dip if galvanizing is being considered. I could go on and on. But I wont. I really like the ABS foam core flooring idea and think this would be something that could be a practical solution to floor rot problems. Fibreglass would be to expensive.

Dan
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Old 06-14-2012, 09:11 AM   #34
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Another issue facing an all aluminum chassis, is how do you fasten the "steel" coupler to the A-frame?

Andy
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Old 06-14-2012, 09:58 AM   #35
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Andy, I'm just thinking out loud here. I think the frames are fine. Armchair design I call it.

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Old 06-14-2012, 11:51 AM   #36
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I was an engineering major for 2 months until I found it interfered with drinking and I became a social science major. Nonetheless, because I was near engineers, I am qualified to guess the answers.

The OP is concerned with bimetal corrosion, but that is easy to solve by isolating the metals from each other. The change I would consider is strengthening the frame so the subfloor is not part of the structure. Then the subfloor would not have to be as strong and other materials may make more sense. Galvanizing the steel makes sense, but anything that costs more than 50˘ is going to be vetoed unless there's a law requiring it. I suspect the European Union requires it. Canada also has stricter standards than the US on some items (I think they require axles with a higher weight rating, for ex.).

I don't know anything about ABS foam core sheets, but ABS plumbing is kind of brittle and can't withstand high heat well. I don't know what temp range of this ABS, but that has to be considered.

I think the belly pan is designed badly and ours is not sealed along the sides so far as I can see, so water can get in. How they keep it warm in the cold seasons with air leaks along the sides is beyond me. The rivets wear through the holes in the pan in several years. The pan is made up of several sections and the black box in the middle may be where the tanks are; that seems sealed well. The rear pan is where the rivets wear through. They used to design cars this badly and the floor boards rusted out.

There's enough expertise around here that we could build better trailers. An engineer with auto/truck experience would be useful for knowledge of seals because cars and trucks don't leak, the 2 Bobs both were in the auto world and know plenty. Rivets are cool, but maybe outdated. If Wally had design patents, they would probably have expired by now (are Silver Streak or Streamline patents available?). All we need is financing and a desire to work. Could we produce them at competitive prices? That would be the hard part. The brands that have come out lately with green or vaguely familiar exteriors just haven't been what most people on the Forum want. Some are very expensive, or interiors not so cool, or QC suspect. Someday, someone will produce a trailer that is more modern in materials and design while looking something like an Airstream and at a better price. Pontiacs, LaSalles and other brands were once cool, but replaced when they got stuck and customers went elsewhere.

Gene
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Old 06-14-2012, 12:26 PM   #37
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Gene, I was able to drink and maintain an engineering major for six years. You obviously did not learn to pace yourself. As any good engineering student will tell you scheduling is very important. I think if you would have stuck it out you would have made an excellent engineer. We would have hired you just to write contracts and requests for change orders requiring paying us large sums of money. I eventually became pretty good at this. But you would have excelled.

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Old 06-14-2012, 01:06 PM   #38
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I find it interesting that the insulated belly pan of an Airstream is occasionally blamed for frame problems. For example, in this thread, there have been the following remarks:

(1981 Airstream) “water under the belly skins. Combine this with the wet insulation and you have a corrosion nightmare.”
(1976 Airstream) “wrapped up with stinky pink insulation so the frames stayed wet with water…. I added belly skin screened drained holes near the leak points (like the step release slot). In my opinion, the belly pan is most of the problem. With it on, it holds water, all but eliminates air flow, and rusts the steel parts.”
(2008 Airstream).”I think the belly pan is designed badly and ours is not sealed along the sides so far as I can see, so water can get in.”

One reason we bought our Airstream was because of the insulated belly pan. Sure helps when the temperatures go below freezing. Should it be watertight? That is probably impossible because of things like the wheel wells. Is there a solution? I think so.

After having our trailer for about a year, I noticed water trapped in the belly pan. My solution was not to seal everything, but to drill a number of 1/8 inch holes in the belly pan at each point where the pan has a low spot. Since then, water has drained out and the insulation has remained dry.

Even with the holes, the insulated belly pan is still effective in freezing temperatures.

Tim
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Old 06-14-2012, 01:27 PM   #39
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Tim, that sounds like a reasonable solution. I wonder why they don't do this at the factory. Maybe someone like Inland Andy can comment on this.

Dan
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Old 06-14-2012, 01:41 PM   #40
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Mine has the pan but I'm not a fan of the pan.
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Old 06-14-2012, 03:05 PM   #41
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I'll echo a few of the comments already made....

First, it's all about the money. To do an aluminum frame right (in my ever humble opinion anyway) you'd do it as a build-up riveted structure. I'd use 2024 or 6061. 2024 is just as strong as the steel in a car frame; 6061 is close but has better corrosion resistance. Airstream could not make money building them like that. But no reason a person with time on their hands couldn't fabricate one. It's more in the labor than the materials,though the materials aren't cheap.

Young's modulus is the elastic modulus of a material; it's "springyness." Yes, aluminum is three times as springy as steel. So you do want to pump up the moment of inertia (essentially the bending stiffness). This is easy to do. And, as we all know, the old long Airstreams are too springy anway. And since aluminum weighs one third that of steel, we can make a much stouter frame and still save weight.

As for fatigue, it is no mystery and is a known quantity. It comes down to cycles. Boeing has an entire book on it that I've read (it's really painful awful boring reading but it's good info and they have it all figured out) There is no way any of us or our descendants will ever cycle an Airstream as much as a jet airliner gets cycled (a cycle is a stress reversal, from compression to tension). Airliners manage to hold together. It's all based on empiral knowledge but basically you have to lower the overall stress levels to achieve higher cycle times before failure. In English, that means you make the frame stronger so that under maximum load it sees a lower percentage of it's total capacity with each stress cycle so it lasts longer. So let's just say that with a certain size trailer and a 4" deep frame of whatever thickness that when you hit the certain size bump, you take the aluminum to 80% of it's yield stress. It's not going to last long like that. But you want the trailer to last forever. So you make the frame 8" deep. All else being equal you just made it 8 times stronger. So now you're hitting about 10% of it's yield stress. You can cycle that a bunch now before it breaks.

So the key is that you cannot simply replace steel with aluminum and keep all the dimensions the same. You have to properly design the frame. But it's not a big deal.

I designed one for my 31' Excella a few years ago. I was going to use steel because it it weldable (if you weld heat treated aluminum, you reduce it to its base state....so if you have 2024 that yields at 40ksi, where you welded it you just reduced its strength to about 10ksi...). I was gonig to make it 8" deep, channel section, 3" flanges, and I believe it was 3/16" thick. It was going to add about 150-175lbs overall weight to the Stream, which I considered negligible and I'd make up for it elsewhere (like throwing the tambour doors over the hill...). I was also going to put a 6" wide perimeter plate all around the outriggers. The U channel would bolt to this plate. The floor would then sit on top of this plate but INSIDE the shell. No more attaching the shell to the floor.

I could bore you with further details, but it was a good design and I still have it in AutoCAD. I even offered it to a guy on here when he asked me about it but he never wrote me back. The only reason i didn't do it was because I was in the middle of building a house and didn't have the time for a full shell off. I sold the Exella and bought an Avion, which has a frame that's actually more stout than the one I'd designed.

Steel does have some advantages here. It's easily repairable, it's durable, it is less prone to fatigue (though we can work around that). It's weldable. And you can get a shop to hot dip your frame once you weld it up.

I agree with the others; I think there are bigger problems that rot them up than the frame being made of steel.

Someday I'll buy two Excella's and cut the front off one and the back off another and make my custom frame and have my Railroad Super Long special with the garage in the middle for the Hog

Until then, see ya on the road!
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Old 06-15-2012, 04:56 PM   #42
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I personally find TIG easier then Gas welding.
I started with oxy acetylene for 4 years so it's my foundation. So I guess it'll always come easiest to me.
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